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“Are your genes to blame if your jeans don’t fit?”

I have recently attended a lecture by Dr. Giles Yeo, a geneticist and researcher at the University of Cambridge. Dr. Yeo addressed a packed audience…

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“Are your genes to blame if your jeans don’t fit?”

I have recently attended a lecture by Dr. Giles Yeo, a geneticist and researcher at the University of Cambridge. Dr. Yeo addressed a packed audience on a Wednesday night to address a topic which garners a lot of interest: why do some people put on more weight (and eat more) than others? What are the role of genes in weight loss? The answer to this question is quite interesting and Dr. Yeo does an excellent job of addressing it in a fun and entertaining manner.

Who has never had that experience of trying to fit into those lovely jeans only to discover that it can’t be done without holding your breath. So who is to blame for that? Was it the Christmas indulgence or above-inflation increases to gym membership?

The good doctor gives us some interesting statistics to consider:
a) an average-sized woman should generally consume around 2000 calories per day in order to survive, while a man would need around 2500 calories. This would translate to 750,000 – 900,000 calories per year.

b) while ageing from their 20 years old to 50 years old an average person will typically gain 15kg in weight on average (with some gaining nothing and others gaining an awful lot more than 15kg).

c) If you and I came with nutritional labels stamped on the collar of our shirts, that label would state that our caloric content would be around 5000 calories per kg.

Surprised girl when she learns the effect of genes on weight loss.

Because of c), the extra 15kg of weight gained over thirty years is worth about 75,000 calories, or 2500 extra calories per year: a day’s ration of calories if you are a man. Doing some math would find that it would only take an extra 7 calories per day over thirty years to put on that additional 15kg in weight! WHAT?!?!

So what does 7 calories look like? Well, it turns out that the closest you can get to that is a 15g serving of ketchup! Yep… mind=blown! Even this would set you back 15 calories. Using the same calculations used previously, an extra squeeze of ketchup or a dip with a chip or fry every day for 30 years would translate to a 30kg weight gain!

So this jaw-dropping calculation begs the question: why are we all not the size of houses?

Dr Yeo to the rescue once again.

Decades of research suggests that mammals will robustly defend their bodyweight. According to Dr. Yeo, the first data came from lab rats. Experiments done in the 1940’s found that if a rat was left in a cage with sufficient food, it would grow at a certain rate. If the amount of food provided to the rats was reduced, they would lose weight (surprise surprise). Once the food provided went back to normal, the rats rapidly started gaining back the weight, but interestingly, only back to the growth rate they were on before the ‘diet’, and they carried on as if no diet had ever happened.

When the food the rats ate was changed to something very tasty with a high content of fat and sugar, then the opposite was true, the rats rapidly gained weight. Once the food went back to normal, the rats’ weight drifted back down to the previous trajectory.

Graph showing the impact of genes in weight loss. Weight deviations from set point tend to correct back to set point.

These experiments led to the development of the ‘set point’ hypothesis, which proposed that all mammals in a stable environment will achieve a genetically determined trajectory of growth. Any deviation from this trajectory will be defended. Data collected since, has shown that the same phenomenon occurs in humans. Despite everything we go through in life, only very unusually will someone deviate more than 20% from that set trajectory over time.

So then, why do we gain weight as we age? There appears to be two reasons:

  1. First, as we go from our twenties to our thirties and beyond, we begin to accumulate inefficiencies in our various organ systems, such that our metabolism begins to slow down. This may occur at different rates in different people, but unfortunately it will happen to all of us.
  2. Second, on average, we move less as we get older. Many factors can influence this such as the nature of the work we are doing. However, we do not tend to reduce the amount we eat.

There are basically two fundamental levers our brains use to defend our bodyweight. Our brain will subconsciously either influence our food intake, our energy expenditure, or both.

Everyone has felt these effects one way or another. Remember a time when you have over-indulged in a feast, perhaps during holiday season. Most will have had that feeling the next day that you can’t eat anything. That is basically your brain subconsciously telling you to take a time-out!

Remember that more than almost anything, your brain absolutely hates it when you lose weight. The amount of fat in your body is essentially how long you would last without food, so the brain equates weight loss with a reduced chance of survival. That is why when you lose a few pounds, your body starts to fight back, lowering your metabolism to consume less energy and simultaneously making you feel more hungry.

Remember that only 7 additional calories have such a material impact over the long term. This is why many dieters experience the frustrating phenomenon of gaining back the weight they arduously fought to lose. Though it might seem you gain back more weight than you lost, that is typically not the case. The weight will revert back to the set point.

Obesity is today one of the greatest public health challenges of our time. The prevalence of obesity has tripled in many countries since the 1980s, according to the Organisation for Economic Cooperation and Development. In the European Union alone there are nearly 150 million adults and 15 million children considered obese.

The issue is not so much with obesity itself but with the accompanying risk to a whole host of diseases, including type 2 diabetes, heart-disease, high blood pressure and certain types of cancer.

So why has bodyweight increased worldwide so fast? Well, our genes have not changed. We have not suddenly evolved as a species and yet, within our lifetime, dramatic changes have occurred. Dr. Yeo says this would put the smoking gun on the guilty hands of environmental changes, an all-encompassing term used to describe changes in lifestyle, diet and working practices.

No matter how much some may protest, the basic physical equation always holds true: the only way you gain weight is if you eat more than you burn. It’s that simple. So the reason why we have become more obese is because we eat too much (of the wrong things) and don’t move enough.

Now, you would expect that if changes in the environment were the only phenomenon at play, everyone’s weight would have changed the same way over time. However, that is not what the data shows. The data points to some people gaining a lot of weight, some people gaining an average amount of weight and some people not gaining a single gram. This means there is something else at play here.

Since we know you gain weight by eating more or by burning less, the question becomes: why do some people eat more than others, why are some people more metabolically efficient and why do some people burn more energy? It is in these questions that lie the biological variability.

Everything in our biology begins with our genes, so to understand biological variation, it is necessary to understand variation in genetic heritability.

Heritability plays a crucial role on how a population looks. Nobody would question the heritability of height. The taller the parents, the taller the children, on average. That seems widely accepted. However, what many people don’t know is that the same is true of weight.

So, while it is true that height and weight are determined by changes in diet, environment and lifestyle, that does not change the fact that if our parents are overweight and/or a certain shape, we are very much more likely to be overweight and/or that shape.

So the truth is that everything about us – our looks, shape, intelligence, risk of mental illness, and millions of other traits – all have a genetic and an environmental component, according to Dr Yeo. It is rare to see, if it exists at all, a trait that is 100 per cent environmental or 100 per cent genetic. The real difficulty is in determining the relative contribution of each and how they interact with each other.

There are many interesting observations about how genetics play a role in body-weight. Two in particular are very informative. The first case involves the Pima Indians who are indigenous native Americans that live in the deserts of Arizona. They hold the title of being one of the heaviest peoples in the world. Nearly all Pimas become obese (many times from a young age) and more than 50 per cent of them suffer from type 2 diabetes. This prevalence of diabetes is nearly 5 times higher than the rest of the United States.

There is however another group of Pimas, those living in the Sierra Madre mountains of Mexico, who display no proclivity to obesity at all. As far as we know, the Pimas were all initially from the Arizona region, but for some reason, at some point in the past, a portion of the population moved to the Sierra Madre region. The two groups of Pima remain genetically indistinguishable, so why would they possess such vastly different body weights and rates of metabolic disease?

The answer lies in the environment. The Arizona Pimas “enjoy” the American lifestyle, together with all of its conveniences and excesses. The Sierra Madre Pimas, however, still live like their ancestors. They are a farming community that still works with animals. Hard labour is a part of daily life and they subsist on a very different diet from their American cousins.

The second example comes from the peoples of the pacific island nations of Polynesia. The small island of Nauru, for instance, is one of the most obese countries in the world, with nearly 95 per cent of its inhabitants being overweight and more than 45 per cent classed as obese. The picture looks the same across these island nations. Why is this the case?

Polynesian man wearing traditional Polynesian clothes. This follows discussion of gene impact on weight loss among Polynesians.

One argument is that in these islands, perhaps more than anywhere else in the world, there is a wide-spread cultural acceptance that bigger is more beautiful. Another argument is that the life of the typical Polynesian is based upon consumption of imported, highly processed food, very little exercise and difficult access to healthcare. Another argument is that the life of the typical Polynesian is based upon consumption of imported, highly processed food, very little exercise and difficult access to healthcare.

These explanations are too simplistic however. There are many cultures where ‘big is beautiful’ and who consume processed food. So why have the Pimas and the Polynesian suffered from that more than the rest?

The underlying reasons are the same, even if the details differ. In both regions there have been millennia of adaptation to fluctuations of food availability in a harsh or geographically isolated environment. Then suddenly, because of factors out of their control, there has been a rapid and dramatic change in the diet and lifestyle of both peoples.

The Pimas have lived on the banks of the Gila and Salt rivers for hundreds of years, long before the Europeans arrived in America. These rivers provided an important lifeline in the deserts of the region. Life would have been very hard, and a genetic premium would have been placed on those individuals better adapted to the episodic ‘feast or famine’ food environment. Individuals adapted to eat and store more fat during periods of plenty would be better placed to survive during periods of famine.

These genes that enabled the strongest to survive would have been passed down the generations, as generations of Pimas undergoing this extreme genetic selection, resulting in a population finely adapted to living in that particular environment.

In the 1920s , however, the US government began a water irrigation project that required the creation of a dam on the Gila river, upstream from the Pima native lands, stopping the flow of water to the region overnight. With a loss of their water and food supply, the Pimas became reliant on government food handouts. These tended to be processed, higher in sugar and refined flour and therefore much more calorically dense – a world away from their traditional diet. The combination of the Pima genes with the new diet resulted in a rapid weight gain for the Pima population.

Colonisation of the Polynesian islands happened across almost three thousand years, with New Zealand being the last island to be colonised. Because of their geographic isolation, the Polynesians would have had to survive canoe journeys of many weeks to those islands as well as be able to survive and thrive in the islands when they arrived. In this context, more body fat certainly correlated with a higher chance of survival.

Polynesian tribe dancing at a sandy beach, wearing traditional Polynesian clothes. This follows discussion of gene impact on weight loss.

Civilisation in the islands evolved to become fishing communities. The arrival of US and British airbases during World War II, as well as the proliferation of mining, drove the import of (highly processed) western food into the islands, initially to fulfil demand from the foreigners, but gradually being adopted by the local population. Again, this was a dramatic change in the environment in a short period of time.

So what is different in the Polynesians and Pimas? Why don’t you and I respond so drastically to processed food? The difference is that the vast majority of us have adapted to living in continental plains. Whenever there was famine, we would simply move somewhere else. However if you were tied to one particular location, the only way to survive would be to evolve to storing more fat.

So what is different in the Polynesians and Pimas? Why don’t you and I respond so drastically to processed food? The difference is that the vast majority of us have adapted to living in continental plains. Whenever there was famine, we would simply move somewhere else. However if you were tied to one particular location, the only way to survive would be to evolve to storing more fat.

The genes that enabled the Pimas and the Polynesians to survive in the past in an environment of ‘feast-famine’ have become deadly in today’s ‘feast-feast’ environment.

If you think of humans in general, we have evolved to do basically two things: eat and sleep. Every waking moment was invested in trying to find what to eat (which involved expending a lot of energy), and when we weren’t doing that, we were mostly sleeping to recover our energies. Today, even though the environment has changed to provide abundant food delivered to our door, we still have these impulses within us.

So assuming the average person with a desk job will have a given energy expenditure on a daily basis, the difference is how hungry each person feels, leading them to eat more or less. And the amount of hunger a person feels is influenced by their genetic makeup.

So to answer the main question in the article: are your genes responsible for you gaining weight? Dr. Yeo concludes that in many ways yes! Over time we evolve to select the genes that would ensure our survival, but similar to what happened to the Pimas and Polynesians, we have never been so exposed to this amount of food as a species before, and that is uncovering traits in our genes which were previously “hidden”.

It is important to remember, however, that no matter what the impact of genes in weight loss is, there is an important case to be made about the quality of the food we ingest and how much we move. Your genes are not a death sentence to your diet, but rather a guiding principle. I will speak more about that in coming posts.

Dr. Giles Yeo is the author of Gene Eating: The Story of Human Appetite. The book is very accessible and highly recommended. You can find Dr Yeo’s book here.

And check out other interesting blog posts here, and guides here.

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Keto Nation

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